Is selling the remaining quantity of school biscuits, which is insufficient for all children, and using the proceeds to buy raw materials for educational aids for the school, while feeling pangs of conscience, considered a sin, and if so, how can one atone for it?

It is not permissible to take what is rightfully due to children and deprive them of it, for Allah the Almighty says: "O you who have believed, do not consume one another's properties unjustly." If the food is not sufficient for them, one can wait until the quota is complete or give one portion to every two students, for Allah the Almighty says: "Allah does not burden a soul beyond its capacity." The responsibility for purchasing raw materials is not yours, but rather that of the responsible authorities. You must purchase the amount of food you sold to distribute it to the students, for the hadith states: "On the hand is what it took until it returns it."